删除xml节点操作问题

从xml文件中删除指定的节点，问题是多次删除操作只有最后一次删除操作才是真正执行的，问下代码哪里有问题？
XML code:

<?xml version="1.0" encoding="UTF-8"?>
<ip-config>
<ip>
<host>221.214.208.17</host>
<port>3128</port>
</ip>
<ip>
<host>222.76.210.8</host>
<port>8080</port>
</ip>
<ip>
<host>222.89.92.106</host>
<port>3128</port>
</ip>
<ip>
<host>218.90.171.249</host>
<port>8080</port>
</ip>
<ip>
<host>218.201.21.177</host>
<port>80</port>
</ip>
<ip>
<host>221.130.13.39</host>
<port>80</port>
</ip>
<ip>
<host>221.130.13.37</host>
<port>80</port>
</ip>
<ip>
<host>221.130.13.232</host>
<port>80</port>
</ip>
<ip>
<host>221.130.13.228</host>
<port>80</port>
</ip>
<ip>
<host>221.130.13.42</host>
<port>80</port>
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